The invention relates to a gasoline internal combustion engine with direct fuel injection into a combustion chamber delimited by a piston top wall. The piston top wall includes a mass electrode movable with the piston toward, and away from, a center electrode. The center electrode extends into the combustion chamber for generating a spark between the center electrode and the mass electrode for igniting the fuel/air mixture generated in the combustion chamber.
Internal combustion engines are generally operated by the combustion of a fuel/air mixture provided in the combustion chambers of the internal combustion engine. The combustion chambers are delimited by pistons longitudinally movably disposed in the engine cylinders. After compression of the fuel/air mixture in a cylinder by the respective piston, combustion is initiated by an electric spark, which is generated between two electrodes extending into the combustion chamber. The ignition voltage is produced by an ignition system and is applied to the electrodes of the spark plug. Provided the fuel/air mixture volume in which the ignition spark is generated is ignitable, that is the mixture composition is within the ignition limits around the stoichiometric point, the mixture volume around the spark will be ignited. The flame so generated ignites the rest of the mixture in the combustion chamber, that is, it initiates the combustion of the fuel starting from the point of ignition thereby increasing the pressure and driving the piston. For a complete combustion of the fuel, which is desirable for achieving maximum engine power output, efficient engine operations and low engine emissions, optimal ignition conditions are necessary. Such conditions are promoted by an easy access of the mixture to the ignition spark (ignition system), by a relatively long spark duration and by a large spark length, that is, a large gap between the electrodes where the spark is generated.
It is possible to provide a variable distance between the electrodes by arranging the center electrode of a spark plug at a particular location within the combustion chamber and provide the mass electrode as a projection extending from the piston top wall. Such a mass electrode is movable with the piston toward and from the center electrode. Depending on the position in the cylinder at the time of ignition, the distance of the mass electrode from the center electrode is different so that different spark lengths are possible. Such an internal combustion engine is known, for example, from WO 87/07682. The engine includes an apparatus for determining the momentary engine load at a particular engine speed. The apparatus assigns a particular ignition time to each operating point of the internal combustion engine. At this particular ignition time, an ignition system generates an ignition voltage between the pair of electrodes of the respective cylinder.
In the lower partial load range of the internal combustion engine ignition occurs at an earlier point in time that is at piston positions clearly before the piston is at the top dead center. In this load range of the internal combustion engine in which the fuel/air mixture is relatively lean, ignition and combustion by means of the two-part electrode arrangement is facilitated at the same time by two factors: 1. the combustion period is relatively long because of the early ignition and, 2. the spark is relatively long since, at the relatively early ignition point, the mass electrode on the piston is still at a relatively large distance from the center electrode. With increasing engine load and increasing engine speed, ignition occurs increasingly later during the compression stroke of the piston. As a result, the mixture is combusted with a combustion pressure suitable for the load of the internal combustion engine. As the piston approaches the top dead center position, also the distance between the electrodes at the time of ignition becomes smaller. The voltage requirements for initiating an ignition spark increases with the engine load, that is, with the combustion chamber pressure. But the voltage requirements decrease when the distance between the electrodes becomes smaller. The voltage requirements remain therefore essentially the same over the whole performance graph range of the engine. As a result, the ignition system needs to provide different ignition voltages only within narrow limits.
However, particularly during partial load operation of the known internal combustion engine, the combustion in some cylinders may be incomplete or the mixture may even fail to be ignited. Ignition occurs sequentially and at equivalent ignition timing for all the cylinders; however, the condition and the stoichiometric composition of the fuel/air mixture supplied to the various combustion chambers may differ. Accordingly, at a particular operating point, some cylinders may have an excessively lean mixture such that the mixture in the particular cylinder cannot ignite.
It is the object of the present invention to provide a gasoline engine wherein a complete combustion is obtained in each cylinder at any engine operating state.